This invention relates to optical waveguide amplifiers, and is particularly concerned with a design of such amplifiers affording the possibility of equalising the gain of such an amplifier at specific wavelengths within the amplification band, those specific wavelengths being ones that individually register with the different wavelengths of a wavelength division multiplexed (WDM) system in which that amplifier is incorporated. In such a system differential amplification is generally to be deprecated because, if the system has a cascade of substantially identical amplifiers, then the differential amplification of the system is equal to the differential amplification of an individual amplifier multiplied by the number of amplifiers in the cascade.
A number of methods have been devised for attempting gain equalisation in an optical WDM system. These include using transmission filters placed within the amplifier, as for instance described by M. Tachbana et al., in a conference paper, paper MD1 of the publication `Optical Amplifiers and their Applications` Vol., 3, (1990). In this method, loss is introduced into each amplifier to reduce the gain in certain spectral regions by the use of an equalising filter. It is thus seen that such equalisation is achieved at the expense of slightly lower pump efficiency. Moreover extra expense and complexity has been introduced into the amplifier design in order for pumping of the amplifier to be arranged to occur in active regions of the amplifier on either side of the equalising filter.
The use of blazed Bragg fibre gratings as transmission filters for this purpose is described in two papers by R. Kashyap et al., respectively entitled `Wideband Gain Flattened Erbium Fibre Amplifier Using a Photosensitive fibre Blazed Grating`, Electronics Letters 21 Jan. 1993, Vol. 29, No. 2, pp 154-6, and `Wavelength Flattened Saturated Erbium Amplifier Using Multiple Side-Top Bragg Gratings`, Electronics Letters 27 May 1993, Vol. 29, No. 11, pp 1025-6.
An alternative method of gain equalisation, that has for instance been described by A. F. Elrefaie et al., IEEE Photonics Tech. Lett., Vol. 15, No. 9, 1026-8 (1993), demultiplexes the signal, uses separate variable optical attenuators for each channel, and then remultiplexes them. Not only is this method lossy and expensive, it also is prone to problems of multipath noise.
A third gain equalisation method, that has for instance been described by V. L. daSilva et al., IEEE Photonics Tech. Lett., Vol. 5, No.4, pp 412-4(1993), and by B. Clesca et al., Post Deadline paper 20, Optical Fibre Conference 1994, utilises gain saturation in an inhomogenously broadened amplifier. In this method the amplifier medium is rendered inhomogenous by selecting a special glass composition, or by cooling the amplifier or by fibre waveguide design. This method has been demonstrated in principle, but in practice uses an unreliable glass composition (ZBLAN) or requires too low a temperature (-200.degree. C.) or has limited applicability.
Some limited measure of equalisation can be achieved by operating different amplifiers in the cascade under different conditions chosen so that some exhibit an spectral gain characteristic that has a positive slope, while others exhibit a negative slope characteristic. This method is for instance described in the conference paper by M Tachbana et al. to which previous reference has been made, and in a conference paper by E Goldstein, Tu 14, OFC 1994. A drawback of this method is that in a erbium amplifier it is applicable to pumping at 980 nm, but not for amplifiers pumped at 1480 nm.
Finally it may be mentioned that the problems attributable to lack of gain equalisation can be reduced by severely restricting the full spectral range of the multiplexed channels, but this is inherently a sub-optimal approach to resolving the problem.